1. Field of the Invention
The present invention relates to a GaN-based compound semiconductor device which exhibits a high breakdown and a good noise immunity and comprises a gate electrode suitable for controlling a large current.
2. Description of the Prior Art
As semiconductor devices suitable for controlling a high voltage and a large current, there are GTO (Gate Turn-off Thyristor) and IGBT (Insulated Gate Bipolar Transistor). GTO, which injects carriers (electrons and holes) both from the anode side and from the cathode side, is characterized by its low ON-state voltage. It should be noted however that GTO requires a protection circuit such as a snubber circuit since GTO suffers from a slow switching speed and a narrow safe operating region in addition to a requirement of a large current for controlling its gate. IGBT, on the other hand, has a fast switching speed, the ability of controlling its gate with a voltage, and a wide safe operating region. IGBT, however, has a problem, resulting from a less amount of carriers injected from the emitter side, in that a higher rated voltage, for example, causes its saturated voltage to suddenly rise to increase a loss of power.
Further, for purposes of readily controlling large power, MCT (MOS Controlled Thyristor), EST (Emitter Switched Thyristor), IGTT (IGBT Mode Turn-off Thyristor) and so on have been proposed. These semiconductor devices, however, are disadvantageous in their low turn-off capabilities and narrow safe operating regions, as is the case with GTO. These semiconductor devices are often implemented as devices of vertical structure using Si-based semiconductor materials.
Nitride-based compound semiconductors such as GaN, AlGaN, InGaAlN and so on have an ON-state resistance during operation smaller by a figure or more than conventional semiconductors such as Si, GaAs and so on, and are capable of operating in high temperature, high breakdown and large current environments. However, a variety of problems remain unsolved for realizing semiconductor devices of vertical structure, for example, GTO and IGBT which are capable of controlling large power using this type of nitride-based semiconductor.
On the other hand, a MOS field effect transistor (MOS-FET) is a semiconductor device which has an insulated gate structure formed of a metal oxide semiconductor. The MOS-FET controls the density of carriers in a channel region below a gate electrode by a field effect to control a current ID flowing between a source and a drain.
For controlling large power by means of the MOS-FET, it is necessary to sufficiently reduce the ON-state resistance between the source and the drain, i.e., the resistance of the channel below the gate electrode. For reference, a reduction in the resistance of the channel only requires to reduce the length L of the channel, increase the width W of the channel, and increase the thickness d of the channel. However, the thickness d of the channel is determined by a voltage applied to the gate electrode and the concentration of carriers in a semiconductor layer in which the channel region is formed, and is in general extremely small, i.e., approximately 1 xcexcm. For increasing the thickness d of the channel, the carrier concentration must be increased, for example, through impurity double diffusion processing, resulting in a more complicated manufacturing process.
To avoid such complexity, it is contemplated to employ the aforementioned GaN-based compound semiconductor for MOS-FET. However, when a GaN-based compound semiconductor is employed, a smaller resistance of a channel causes inconvenience such as a tendency to malfunctions due to noise. In this respect, in an Si-based MOS-FET, a source region is short-circuited to a substrate in which a channel region is formed to form a diode between a source and a drain, thereby ensuring a noise immunity. However, the GaN-based compound semiconductor still implies a problem as to how a diode is formed.
It is an object of the present invention to provide a GaN-based compound semiconductor device using a GaN-based compound semiconductor having a high ON-state resistance during operation, which exhibits a high breakdown and a good noise immunity and is capable of operating with a large current.
To achieve the above object, the present invention particularly provides a GaN-based compound semiconductor device having a semiconductor layer immediately below a gate electrode, which is formed of a semiconductor material having a larger band gap than semiconductor materials forming other semiconductor layers, to realize a GaN-based compound semiconductor device having a gate electrode such as GTO, IGBT and so on.
Specifically, the present invention provides a GaN-based compound semiconductor device which employs AlGaN as a semiconductor layer immediately below a gate electrode.
More specifically, the GaN-based compound semiconductor device according to the present invention employs GaN-based semiconductors for n-type layers which make up a field effect transistor having a gate electrode. Then, the GaN-based compound semiconductor device is characterized in that a p-type GaN-based semiconductor layer is formed in the n-type layer for serving as an electron amplifier layer, and an insulating layer made of AlGaN or the like having a band gap larger than an insulated gate layer is provided for the p-type layer. When the GaN-based compound semiconductor device is not applied with a gate bias, no current is allowed to flow due to its pnp structure. On the other hand, the GaN-based compound semiconductor device is applied with a positive gate bias, which is more positive than a voltage between a source and a drain, to generate n-type carriers due to the field effect on the interface between the p-type layer and the insulating film, thereby causing the semiconductor device to switch on/off between the source and the drain.
Preferably, in this event, to increase the area of a gate region to enable the GaN-based compound semiconductor device to control a large current, a groove may be provided along the center of a cathode to form a gate having a large area in the groove. Also, a GaN multi-layer film of pnpn structure may be formed on a conductive Si substrate or SiC substrate, such that a groove is formed in the GaN layer by dry etching or the like to form a gate electrode in the groove. Further, in IGBT and IEGT, an oxide film may be formed below the gate such that a gate electrode is formed thereon.
According to the GaN-based compound semiconductor device having the device structure as described above, since the semiconductor layer immediately below the gate electrode has a larger band gap than the remaining semiconductor layers, carriers can be effectively injected, with an inversion layer formed in the semiconductor layer immediately below the gate electrode. As a result, it is possible to realize a GaN-based compound semiconductor device such as GTO, IGBT and so on which has a high breakdown and is capable of operating with a large current.
It is another object of the present invention to provide a GaN-based compound semiconductor device which has a short channel length by reducing the length of a semiconductor layer near a gate, in which a channel region is formed, to reduce its ON-state resistance, and a pn junction diode formed to be connected in parallel between a gate electrode and a source electrode in semiconductor layers which constitute an insulated gate structure.
Particularly, the present invention provides a GaN-based compound semiconductor device comprising a plurality of semiconductor layers formed to constitute an insulated gate structure, wherein the semiconductor layers includes a first semiconductor layer made of a GaN-based semiconductor having a low impurity concentration, a second semiconductor layer made of a GaN-based semiconductor having a conductivity opposite to that of the first semiconductor layer and a high impurity concentration, and embedded in the first semiconductor layer, a third semiconductor layer made of a GaN-based semiconductor having the same conductivity as that of the first semiconductor layer and a high impurity concentration, and embedded in the second semiconductor layer to comprise a source region and to define the length of a channel region formed in the second semiconductor layer, and a fourth semiconductor layer made of a GaN-based semiconductor having the same conductivity as that of the first semiconductor layer, and embedded in the first semiconductor layer spaced apart from the second semiconductor layer, or located on a back surface of the first semiconductor layer to form a drain region. In the plurality of semiconductor layers constituting the device structure, a pn junction diode is formed as connected in parallel between the gate electrode and the source electrode.
Preferably, the present invention provides a GaN-based compound semiconductor device which has the pn junction diode formed between the third semiconductor layer forming the source region and the fourth semiconductor region.
Also, the present invention provides a GaN-based compound semiconductor device which further comprises a fifth semiconductor layer made of a semiconductor layer having a conductivity opposite to that of the second semiconductor layer, and embedded in a region beside a channel region formed in the second semiconductor layer, and a sixth semiconductor layer made of a semiconductor layer having the same conductivity as that of the second semiconductor layer, and embedded in the fifth semiconductor layer and connected to the gate electrode, wherein the pn junction diode is formed between the fifth semiconductor layer and the sixth semiconductor layer.
According to the GaN-based compound semiconductor device having the device structure as described above, a semiconductor device has an insulated gate structure of short-channel type and a pn junction diode acting in parallel between a source and a drain, thereby making it possible to realize a semiconductor device which is highly immune to noise and suitable for controlling a large power, making use of the advantage of the GaN-based compound semiconductors which have a high breakdown and a small ON-state resistance.